Biomedical Engineering Reference
In-Depth Information
nanoscale imaging), nor can they be accomplished easily with sucient accu-
racy. Furthermore, additional issues with ex situ characterizations can also
emerge. For instance, misleading results associated with exposure to the ex
situ environment during sample preparation or characterization may inhibit
interpretations of the underlying surface or interface phenomena. Facing these
problems, one shall find that in situ characterizations are thus more desirable,
critical, and necessary for defining the properties of the porous media used in
such applications.
These porous media can be categorized into inorganic, organic (most likely,
polymeric) materials, and hybrid composites. Porous silicon oxides and metal
oxides are examples of the inorganic materials. Conductive (electronic and/or
ionic) polymers are usually referred to as the organic materials. Polymer-
carbon nanotube composites are a good example for the hybrids. These materi-
als are usually used as the backbone for immobilization of biocatalytic species,
including microbial, enzymatic catalysts, cofactors, or mediators. They can
also act as the current collector for mediators or microbial cells to enable elec-
tron transfer or as membranes to separate reactants to prevent interference in
electrode reaction or fuel crossover (and thus eciency loss). To characterize
these materials for their feasibility in the biofuel cell applications, engaging
in situ observations can be advantageous to help us monitor and understand
their behavior under operating conditions. Such noninvasive, in situ obser-
vations remain challenging yet rewarding with great payoffs. In this chapter,
some recent advancement in the in situ characterization techniques, primarily
spectroscopic in nature, are reviewed to provide some introduction of how they
can be utilized in the understanding of porous media applications in biofuel
cells.
A variety of in situ characterization techniques can be utilized particularly
for studying small quantities or sizes of porous materials or conductive mem-
branes, usually in thin film forms. Applying such techniques, often in a liquid
environment, does require some nonconventional approaches to characterize
the pore structure and related (either physical or chemical) properties. Par-
ticularly interested are approaches incorporating various techniques includ-
ing spectroscopic imaging ellipsometry (SIE), quartz crystal microbalance
(QCM), x-ray diffraction, fluorescence, and reflectometry (XRD, XRF, and
XRR), or laser scanning confocal microscopy (LSCM), in combination with
electrochemical techniques to yield insightful information of porous media's
unique properties in solutions and at interfaces. These techniques are either
surface or bulk sensitive, or both; and, they can provide either spatial or
temporal information simultaneously or separately to reveal the dynamic
nature of the processes in a biofuel cell, or on an electrode surface, or in
the membrane. Combining the information obtained from these in situ tech-
niques carefully, a wide spectrum of data can be obtained to achieve sucient
understanding of the electrode or cell behavior to allow us to clearly identify
cell operating limitation, understand limiting mechanism, and improve cell
performance.
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